If those LUA parsers implementations are incorrectly done in terms of trying to generate too much meta or incorrectly tagging large field values to a meta, that could be the reason to grow index chunk size.
As an example, you may also observe that the following meta(s) in each index slice show a large size.
sld is a default OOTB meta named Second Level Domain and this is a text field. So the factors influencing the size would be the valueMax parameter used in the index-concentrator-custom.xml file. Also if there is a custom parser or custom implementation of the OOTB parsers which is badly written, then it could cause large text to be tagged to this meta, which could also increase the size.
The word meta is consistently 17MB in size, which basically implies that there seem to be parsing-related issues which are causing many word meta(s) being generated. Normally if the parsers are well written, only a few word meta(s) might get generated.
To reduce index chunk size, you need to reduce the /index/config/save.session.count config value from auto(defaults to 200 million) to 100000000(100 million) and it depends on the customer's environment. This might have a slight performance penalty in terms of the queries opening a few extra index slices but opening one file of 11+GB in memory is more problematic than opening 3 files of around 4 GB in size. you can look up /index/stats/sessions.since.save stat value and see how far off from 200 million it basically is. For your information, you can do a manual index save, (concentrator->config->index->save) in case you want to save index chunk data.